1. Field of the Invention
The present invention relates to fiberboards and manufacturing methods therefor.
2. Description of the Prior Art
It is quite a difficult problem to ensure a stable supply of plywood for a long time to come.
In recent years, in association with environmental problems such as global warming and desertification, there has been a tendency that regulations against deforestation are tightened principally for tropical rain forests. On this account, in place of plywood made primarily of round timber, more attention has been paid to particle boards, strand boards, medium density fiberboards or other wood-based boards which employ wood particles or wood fibers as their material.
These boards are produced by mixing wood particles or wood fibers with adhesive and then hot-pressing the mixture. Unlike plywood, round timber of large-diameter are not necessarily involved in those boards and small-diameter woods may be used therefor. Besides, in some cases, miscellaneous trees, woodwork trash, waste woods, defective woods and the like can be utilized, so that those boards have an advantage that effective use of raw material can be made.
Among others, medium densitiy fiberboards are fiberboards produced by board-making fine fibers obtained from wood, so that the medium density fiberboard is superior in processibility and surface smoothness. Moreover, in terms of cost, medium density fiberboards are lower in price than plywood. Thus, medium density fiberboards have begun to be used in a wider range of fields typified by furniture or the like.
Wood fibers to be used for medium density fiberboards are obtained by processing small particles of softwood or hardwood into fibers having a length of, usually, 6 mm or less with a refiner, a defibrator or other defibrating machines. When surface smoothness or processibility is required, short fibers of 2 mm or less are frequently used.
The strength of a fiberboard depends on the strength of the fiber itself, intertwinement of fibers, strength of bonds between fibers and the like. For the existing medium density fiberboards, which use short fibers of a few mm or less, the intertwinement of fibers less contributes to the strength of the fiberboard, while the strength of the bonds between fibers contributes rather than the strength of the fiber itself makes a greater contribution.
Further, with fine fibers, numerous bonds between fibers are involved so that ordinary adhesive process could hardly achieve a strong bonding between many fibers. To enhance the strength of adhesion, one way would be to increase the amount of adhesive and thereby strengthen the bond between the fibers. In this case, however, large amounts of adhesive would be necessitated to attain a strong bond between fibers, hence being impractical in terms of cost. As a result, there is a limitation also in enhancing the strength of adhesion, so that the strength of the fiber itself is not reflected sufficiently. Consequently, medium density fiberboards are lower in mechanical strength than plywood.
Also, wood fibers which are a material of medium density fiberboards would vary in dimensions upon water absorption or moisture absorption. On this account, an medium density fiberboard formed from wood fibers would yield a dimensional change within a plane parallel to its surface upon water absorption or moisture absorption. Furthermore, since an medium density fiberboard has been strongly compressed thicknesswise during the pressing process, the medium density fiberboard would recover from the compression on account of water content, yielding a large expansion in thickness upon water absorption or moisture absorption. As a result, the medium density fiberboard is inferior also in dimensional stability to plywood.
Meanwhile, attempts have been started to utilize, as building materials, unused plant resources such as palm fibers which have been waste materials hitherto, instead of using the above wood resources. Such attempts are described in Japanese Patent Laid-Open Publication HEI 09-94811 (palm shell mat) and the like.
For building members utilizing unused plant resources, which are described in these prior arts, fibers obtained from unused plant resources are randomly arranged and intertwined so as to make up a mat form. Those building members are intended for use primarily as core material for tatami mats, cushioning material, heat insulating material and the like.
These mat-formed bodies, being low in density and having numerous voids inside, have such features as light weight and superiorities in air permeability, moisture permeability, cushioning property, sound absorbency, heat insulating property and the like. However, because of the fact that those bodies retain the strength depending mainly on only the intertwinement of fibers, as well as the fact that they have numerous voids inside, the mat-formed bodies are inferior in strength to plywood, medium density fiberboards and the like, thus inapplicable to building materials such as floor material, wall material and roof material.
As described above, there have been growing needs for fiberboards which are comparable with plywood in terms of fundamental performances of boards such as strength and dimensional stability, taking into consideration the viewpoints of global environmental problems and effective use of wood resources, and which are low priced in terms of cost.